I don't know why people think "Blaine" was a fan mistranslation. If anything, "Brain" was an SENA mistranslation. The "l" is replace with "r" in Japanese-spoken English words because that's the style. If the original was "Bu-rain-u" or something like that, then it would be natural that the English translation was "Blaine". Again, "Brain" was probably another SENA translation error (right Roxas).

Maybe they can be both correct translations, because in japanese Blaine and Brain sound the same? I don't know japanese, I'm just guessing, but if this is the case then there's no mistake, just a choice they made.